1. Field of the Invention
The present invention relates to a data communication system and a data communication method, and, in particular, to a data communication system for an intermittent activation control circuit and a data communication method in the same.
2. Description of the Related Art
In recent years, safety-related regulations have been tightened in Japan and US. According to the TREAD Act (Transportation Recall Enhancement, Accountability and Document Act) that will come in effect in North America, a tire pneumatic pressure monitoring system should be installed to new cars to be on the market in 2006 and afterward. On that account, studies are currently being performed on a technique of attaching a sensor inside a tire and measuring a pneumatic pressure and temperature of the tire. In the technique, a sensor unit is installed in a valve portion of each of four tires to monitor all the four tires independently. This technique offers advantages in that it is possible to monitor the tire pneumatic pressure in a high precision even while the car is stopped or parked.
As a pneumatic pressure monitoring system, a system is known in which a tire pneumatic pressure is measured in a specific time interval, data on the tire pneumatic pressure is sent to a car body by radio wave, and the data is displayed on a display unit provided to a front panel of the car. This system is provided with a transmitting module installed inside the tire wheel and a receiving module attached to the car body. The transmitting module is provided with several kinds of sensors for detecting a pneumatic pressure, temperature, etc., a receiving section for receiving a command data sent from the receiving module by LF (Low Frequency) wave, and a transmitting section for transmitting data obtained by the sensors to the receiving module by RF (Radio Frequency) wave. Electric power consumed in the transmitting module is all supplied from a battery connected to the transmitting module.
As mentioned above, the transmitting module is attached inside the tire wheel mainly carries out tire pneumatic pressure measurement, signal processing, radio transmission, etc. Thus, the transmitting module is required to be compact and lightweight so as not to affect tire balance. For this purpose, it is practical with respect to a cost that the battery used for the module is a button battery or the like. Also, the battery must be connected to the transmitting module in such a manner as not to be easily disconnected due to vibration on driving. Generally, the battery is not replaced until the tire is discarded.
Under these use conditions, it is desired to utilize the transmitting module efficiently in minimum power consumption.
In conjunction with the above description, a “device for detecting tire mounting position” is disclosed in Japanese Laid Open Patent Application (JP-P2003-220809A). In this conventional example, a tire pneumatic pressure detecting section is installed inside each tire to detect a pneumatic pressure of each of tires. A transmission section is installed inside each tire together with the tire pneumatic pressure detection section, and transmits tire pneumatic pressure data detected by the tire pneumatic pressure detection section together with a tire identifier. Receiving sections are installed in the vehicle body in proximity to the tires to receive the tire identifier and the tire pneumatic pressure data transmitted by the transmission section. The transmission section has a wake mode in which a signal is transmitted to the receiving section and a sleep mode in which no signal is transmitted. The receiving section, at a time of the driving of the vehicle, issues a wake-up signal to the transmission section installed in the tire closest to the installation position of the receiving section to switch the transmission section from the sleep mode to the wake mode. Then, the receiving section receives the tire pneumatic pressure data and the tire identifier returned from the transmission section, and confirms the tire mounting position based on the tire pneumatic pressure data and the tire identifier.
In the above-mentioned conventional example, when the transmission section is in the sleep mode, the receiving section cancels the sleep mode by transmitting a wake-up signal to the transmission section. Accordingly, a part of the transmission section must keep its active state so as to be always able to receive the wake-up signal from the receiving section. Thus, a current is consumed all the times. As a result, the lifetime of a battery becomes short in the above-mentioned system in which it is difficult to replace the battery. As a result, a problem is caused that a predetermined operation time, e.g., ten years is not satisfied. In addition, a cost increases if a large-sized battery is used to secure the predetermined operation time.